The present invention relates to an apparatus for controlling the speed of sewing by applying a speed instructive pulse signal to the sewing machine's driving and braking mechanism.
Conventional sewing machine speed control apparatuses have been provided with a speed detector, a counter for counting the number of pulses generated by the speed detector at an interval proportional to the sewing machine's rotational speed, a control circuit for producing a speed instructive pulse signal to a driving means (such as a clutch motor) or a braking means (such as an electromagnetic brake) of the sewing machine in accordance with the pulse count, an oscillator for setting the timing for applying the pulse count to the control circuit, and a speed setting circuit for setting the rotational speed of the sewing machine, and for outputting the set speed to the control circuit. Sewing machine speed was controlled so that, at the same time that an oscillator pulse was applied to the control circuit, the counter content was applied to the control circuit. The control circuit would compare the counter content with the instruction from the speed setting circuit and provide an instruction to the driving means or the braking means.
However, in the conventional sewing machine speed control apparatus, it was impossible to control the driving means and the braking means more finely than the interval of pulses of the oscillator would permit, because the application of the output of the counter to the control circuit was controlled by the synchronization provided by the oscillator. As a result, even when the difference between the speed set by the speed setting circuit and the actual running speed was very small, the content of the counter was not entered into the control circuit during the oscillation period of the pulse, so that the previously set speed was continuously applied until the next pulse was produced, so that ripples in the speed became large.